Traditionally, antenna arrays at base-stations were constructed by means of passive fixed beam antennas each driven from a single radio transmission/reception unit. However in recent years, technological advances mean that advanced base stations can be equipped with an array of antennas where individual antennas or subgroups of antennas are each driven with their own radio transmission/reception unit. Such arrangements are referred to as “Active Antenna Systems”, or AAS.
A functional overview of an AAS base station is provided in FIG. 1. The base station comprises a number of transceiver units TXU/RXU 1, TXU/RXU 2, . . . , TXU/RXU K, in FIG. 1 depicted as transceiver array 11. Each transceiver unit is mapped onto one or more physical antennas A11 . . . Amn by means of a radio signal distribution network (RDN) 12.
AAS offers flexibility to optimize radio network performance by means of a variety of potential applications. These include but are not limited to variable electronic downtilt, cell splitting, user specific beamforming and spatial multiplexing. Applications may each involve “beamforming”. Beamforming, also denoted spatial filtering, is a signal processing technique used in sensor and/or transmitter arrays for directional signal reception or transmission. This is for example achieved by combining elements in a phased array in such a way that signals at particular angles experience constructive interference while others experience destructive interference, thereby achieving spatial selectivity. The improvement compared with omnidirectional reception/transmission is known as the receive/transmit gain (or loss). Beamforming is achieved by individually modifying the phase and the amplitude of the signal that is transmitted from each of the different transceivers.
In any cellular system, an important parameter relating to the transmitter system is the quality of the transmitted signal. If the quality of the transmitted signal is imperfect, then in addition to the wanted in band signal a certain amount of distortion in band components will be transmitted. At the receiver, the relative RX power levels of the wanted signal and the distortion components will not vary according to the wanted signal RX power level. Thus, when the Signal to Interference and Noise level (SINR) at the receiver is large, then the distortion components may become a limiting factor in demodulation performance.
Transmitter induced distortion can arise from many sources such as phase error, PA (Power Amplifier) non linearity, transmitter noise etc. A very significant source of distortion is due to so-called clipping, in which the peak power of the transmitter signal is limited in order to limit the peak to average power ratio at the power amplifier. Avoiding a large peak to average power ratio is essential for achieving an economic power amplifier design with low distortion. However limiting the transmitted signal in this way causes transmitter induced distortion.
Transmitter induced distortion due to peak power reduction schemes can show a large spatial fluctuation in AAS (Active Antenna System), which will significantly reduce performance in some areas of the cell and can cause significantly increased implementation costs.
An AAS system must also meet a requirement on the quality of the transmitted signal. The requirement must be met at each point in space at which a scheduled UE receives the signal from the base station.
One existing solution is to set a requirement on the maximum EVM (Error Vector Magnitude) at the antenna connector. AAS systems consist of possibly multiple antenna connectors. A first problem with the existing solution is that antenna connectors may not be available in an AAS system. However assuming that the AAS can be controlled at the antenna connector and meet a requirement, there exists a further problem due to beamforming. The beamforming involves modifying the phase and the amplitude of the signal transmitted through each radio transmitter.
Clipping, as mentioned above and further described below must be applied at each radio transmitter of the AAS. The clipping noise is in phase with the signal, and thus the phase element of the beamforming will apply to the clipping noise in addition to the transmitted signal.
However the relative level of clipping compared to the wanted signal will depend on the amplitude of the signal. Thus when amplitude weighting is applied to the signal that is different at each transmitter, the power of the clipping signal relative to the wanted signal will differ at each of the transmitters given uniform clipping thresholds. This will have the effect of causing the spatial characteristics of the radiated clipping signal to differ to those of the wanted signal.
A UE that is within the main beam of the wanted signal may experience a reduced distortion level from the clipping, and hence experience a lower EVM than required. However a UE that is within a side lobe or null of the wanted signal that does not correspond to a side lobe or null of the clipping signal may experience very poor EVM.
A possible but inferior solution to this problem is to tighten the EVM requirement at each transmitter antenna connector such that even at the point in space at which the received EVM is at its worst, the existing requirement is met. This would however imply a very tight requirement on each transmitter, which may be difficult or impossible to meet and/or imply very high cost.